A typical power distribution system comprises a generator connected via some distribution network to one or more loads. The generator is typically a multiphase generator and the voltage output on each phase is nominally sinusoidal. If the generator supplies a purely resistive load, then the current drawn from the generator will also be sinusoidal, and is also in phase with the voltage. If the generator is connected to a load with a significant reactive component then the relationship between the current and voltage becomes altered and the current starts to move out of phase with the voltage thereby giving rise to additional distribution losses and also de-rating the maximum capacity of the power source. Furthermore, if the load is non-linear then it tends to draw a distorted current (i.e. not sinusoidal current) from the generator. These distortions can give rise to harmonic variations in voltage which are seen by other loads connected to the generator. These distortions may affect the performance of these other loads, as well as degrading the performance of the power supply system. Thus it is well known that it is desirable to keep harmonic distortion low.
Aeronautical systems represent a particular challenge to control of harmonic distortion. This is because in some modern aircraft the generators typically operate over a wide frequency range, and thus methods of reducing harmonic distortion using filters that work well with fixed frequency systems cannot be used in aeronautical systems. Furthermore, many of the flight surfaces may be electrically actuated and these actuators can by virtue of their variable speed drives or power supply arrangement impose significant non-linear electrical loads. Finally, solutions for providing control of harmonic distortion often involve the use of transformers. These transformers have to be rated to be able to handle the maximum power that is to be transferred across them together with an acceptable margin of safety. Transformers capable of handling large amounts of power tend to be relatively heavy.
Variable speed controllers and DC converters are often based on a three phase bridge circuit, an example of which is shown in FIG. 5. Such a bridge circuit is also known as a 6 pulse bridge because there are six pulses per cycle—this being formed on one per half cycle for each of the three phases.
In theory a six pulse bridge circuit produces current harmonics at 6n+1 and 6n−1, where n is an integer, i.e. at 5, 7, 11, 13, 17, 19 and so on times the fundamental frequency. Also the magnitude of each harmonic is approximately a reciprocal of its harmonic number. Consequently there would be 20% fifth harmonic, 14% seventh harmonic, 9% eleventh harmonic and so on.
In principle there are no even-order harmonics because the bridge circuit is a full wave rectifier.